This invention relates to the reproduction of a digitally encoded signal from a medium and in particular to the selection of reproduced data for subsequent processing.
The introduction of disks recorded with digitally compressed audio and video signals, for example, utilizing MPEG compression protocols, offers the consumer sound and picture quality virtually indistinguishable from the original material. However, consumer users will expect such digital video disks or DVDs to offer features similar to those of their analog video cassette recorder or VCR. For example, a VCR may reproduce in either forward or reverse directions at speeds other than the recorded speed. Such non-standard speed playback features are also known as trick play modes. The provision of trick play features are less easily provided with MPEG encoded video signals due to the hierarchical nature of the compression which forms pictures into groups having varying degrees of compression. These groups are termed groups of pictures or GOPs, and require decoding in sequence. A detailed description of the MPEG 2 standard is published as ISO/IEC Standard 13818-2. However, in simple terms, an MPEG 2 signal stream may comprise three types of pictures having varying degrees of content compression. An intra-coded frame or I frame has the least compression of the three types and may be decoded without reference to any other frame. A predicted frame or P frame is compressed with reference to a preceding I or P frame and achieves greater degree of compression than an intra-coded frame. The third type of MPEG frame, termed a bi-directionally coded or B frame, may be compressed based on predictions from preceding and/or succeeding frames. Bi-directionally coded frames have the greatest degree of compression. The three types of MPEG frames are arranged in groups of pictures or GOPs. The GOP may for example contain 12 frames arranged as illustrated in FIG. 1A. Since only an intra-coded frame is decodable without reference to any other frame, each GOP may only be decoded following the decoding of the I frame. The first predicted frame or P frame, may be decoded and stored based on modification of the stored, preceding I frame. Subsequent P frames may be predicted from the stored preceding P frame. The prediction of P frames is indicated in FIG. 1A by the curved, solid arrow head lines. Finally, bi-directionally coded or B frames may be decoded by means of predictions from preceding and or succeeding frames, for example, stored I and P frames. Decoding of B frames by predictions from adjacent stored frames is depicted in FIG. 1A by the curved, dotted arrow head lines.
The hierarchical nature of the coded frames comprising MPEG groups of pictures necessitates that the I and P frames of each GOP are decoded in the forward direction. Thus, reverse mode features may be provided by effectively jumping back to an earlier, or preceding I frame and then decoding in a forward direction through that GOP. The decoded frames being stored in frame buffer memories for subsequent read out in reverse to achieve the desired reverse program sequence. FIG. 1B illustrates play back in the forward direction at normal speed and at a time prior to time to, a reverse three times speed mode trick play mode is selected. The trick play mode is initiated at time t0 where I-frame I(25) is decoded and displayed. The next frame required for decoding is I-frame I(13), thus the transducer is repositioned, as indicated by arrow J1 to acquire frame I(13). Having recovered and decoded I-frame I(13), the transducer tracks, as indicated by arrow J2 to acquire and decode frame P(16). The process is repeated as indicated by arrows J3, J4. Following the acquisition and decoding of frame P (22) the transducer is moved as depicted by arrow Jn to recover frame I(1). To smoothly portray scene motion requires the decoding and display of I, P, and possibly B-frames. The jump and play process is repeated for preceding GOP, thereby progressing haltingly backwards through the records whilst smoothly portraying the program material in a reverse sequence at the video output.
The transducer or opto-pickup is servo controlled to follow the recorded track and to maintain optical focus. In addition the transducer may be repositioned or jumped to a specific sector of the recorded track responsive to a sector address coupled to the transducer control servo system. Such a transducer jumps may result from parental guidance selection, alternative angle selection, user searching or trick mode reproduction. During transducer repositioning the reproduced bitstream will disappear and the error correction buffer will contain gaps. However such gaps are of short duration and are flagged by a data valid signal. The transducer is quickly repositioned and refocuses to acquire data from the recorded track, however, the recovered data may precede that requested since it was transduced from sectors occurring possibly one revolution before the wanted sector address. This acquisition of unwanted data results as the disk rotates to position, or approximately position, the wanted sector under the transducer. Thus, although the transducer is repositioned, the error corrected bitstream 41 coupled to the back end initially includes data from unwanted preceding sectors which must be identified by a microcontroller and discarded. Such processing of unwanted replay data represents unnecessary, additional microcontroller and buffer memory utilization.
In an inventive arrangement, unnecessary processing of unwanted sector data is avoided. A method for controlling data reproduced in sectors by a disk player employing optical read out, comprises the steps of transducing groups of sectors including sectors wanted for processing, and sectors unwanted for processing. Supplying the wanted sectors exclusive of the unwanted sectors to a data processor for processing, and processing the wanted data sectors to extract data therein representative of video information.